1. [Field of the Invention]
The present invention relates to an ultraviolet screening composite oxide and a process; for producing the same. More particularly, the present invention relates to a white ultraviolet screening composite oxide which is used in the preparation of paints, inks, cosmetics and the like, excellent in the capability of screening ultraviolet rays, highly visible-light transmittable and excellent in storage stability.
2. [Prior Art]
Ultraviolet rays having a wavelength of 320 to 400 nm get the skin sunburnt and are causative of melanism, inflammation, etc. Further, the ultraviolet rays often decompose a matrix resin contained in paints and cosmetics, decompose or fade a pigment, or oxidize fats and oils and perfume to bring about the deterioration and change of flavor thereof.
For this reason, an attempt to solve these problems has been made through the use of an ultraviolet screening material. An ultraviolet screening material of this type known in the art is ultrafine titanium oxide. Although titanium oxide exhibits an excellent screening effect in the region of ultraviolet rays, it is poor in transmittance in the region of visible lights because the refractive index (2.61 to 2.90 in the rutile form) of titanium oxide is larger than that (2.00 to 2.02) of zinc oxide. For this reason, the incorporation of titanium oxide in transparent materials, such as paints and cosmetics, causes them to turn opaque white in color. Further, titanium oxide is disadvantageous because it is liable to deteriorate the matrix upon being exposed to ultraviolet rays as compared with zinc oxide.
It is reported that ultrafine zinc oxide may be used as another screening material. Specifically, Japanese Patent Appln. Laid-Open Gazette No. Hei. 2-208369 (208369/90) proposes the use of ultrafine zinc oxide having a specific surface area of 20 m.sup.2 /g or more as an ultraviolet screening material, and makes mention of the screening effect in the ultraviolet region and the transmittance in the visible region. Even when such an ultrafine zinc oxide having a large specific surface area was used, the transmittance in the visible region was unsatisfactory.
Japanese Patent Appln. Laid-Open Gazette No. Sho. 62-275182 (275182/87) disclose an ultraviolet screening material (or agent) composed of a composite metal oxide in which the metal is aluminum, iron or the like and zinc. This ultraviolet screening material exhibits excellent ultraviolet screening capability and high visible light transmittance. However, although this screening material is superior in such screening capability and transmittance as above at its initial stage, it will subsequently raise a problem as to its superior properties being deteriorated with time thereby making its storage stability inferior. Accordingly, the above ultraviolet screening material raises a problem that it cannot satisfactorily be used over a long period of time and is not suited for practical use.